The present invention relates to a circuit arrangement and a method for determining the load current through an inductive load connected to a supply voltage in a clocked manner.
There are a wide variety of applications in which inductive loads, such as, by way of example, electric motors or electrically actuable valves, are connected to a supply voltage in a clocked manner according to a pulse-width-modulated drive signal in order thereby to regulate the current consumption of the load. In the case of electric motors, by way of example, the rotational speed of the motors can be regulated by way of the duty ratio of the pulse width modulation and the current consumption dependent thereon. For suitable driving of the load, it is necessary in this case to detect the load current flowing through the load.
FIG. 1 shows an example of the interconnection of an inductive load L for pulse-width-modulated connection of the load to a supply voltage. The load L is connected in series with a switch S, which is driven according to a pulse-width-modulated signal PWM, between a terminal for supply potential Vbat and a terminal for reference-ground potential GND. In order to detect a load current flowing through the load L, provision is made of a current measuring impedance Z connected between the load L and the switch S. The switch S is connected between the load L and reference-ground potential GND; a freewheeling diode D is connected in parallel with the series circuit formed by inductive load L and measuring impedance Z.
In the case of the circuit arrangement illustrated, it is possible to tap off a voltage Vm across the measuring impedance Z, which, according to Ohm's law, is related to the current flowing through the load L by way of the value of the measuring impedance Z. In the case where switch S is closed, disregarding a voltage drop across the switch S, said voltage Vm is referred to reference-ground potential GND and can be evaluated by means of a simple differential amplifier (not illustrated). After the switch S has been opened, a current continues to flow through the measuring impedance Z, but now said current flows via the freewheeling diode D. The measurement voltage Vm is now referred, however, to a potential that lies above the supply potential Vbat by the value of the forward voltage of the diode D. Evaluation of such measurement voltages referred to a high potential necessitates special amplifiers, so-called rail-to-rail amplifiers, which are able to evaluate voltages that are referred to greatly fluctuating reference potentials. However, such amplifiers are complicated to realize and therefore cost-intensive, for which reason one has hitherto been restricted in many cases to detecting the current through the inductive load L only during the switch-on phase of the switch S.
It is an aim of the present invention to provide a circuit arrangement and a method for determining the load current through an inductive load connected to a supply voltage in a clocked manner which can be realized simply and cost-effectively and provide a measured current value both during the switch-on phases, in which the load is connected to the supply voltage, and during the switch-off phases.